Actually its not exactly dead center, it mounts the spur in the lower half of the opening, closer to the motor.
Dave is right, the R1 plastic shaft holder will work but be careful as it is very flexy.
The whole point of the R2 using the two bearings is to minimise play so use the R1 holders at your own risk
Come to think of it, the R2 holders are also very flexy. This is why I highly recommend the alloy center shaft holders as a 1st priority hop-up.